I just bought a '93 RM 125 that wasn't running. The guy said it just quit on him and he parked it a couple months ago. I tore it down to the frame last night, and today took the top end apart. The piston was missing a large portion of the skirt on the exhaust side. I found the missing piece lodged in the reed valve. What might cause the piston to crack like that? I measured it, but don't have the number in front of me, but it was a little lower than the limit according to the Clymer. Suprisingly the cylinder doesn't really have any bad marks, except right at the top there is a ridge all the way around that you can feel with a fingernail, but I assume that's where the top of the piston ends its stroke. The last time I rebuilt a dirt bike top end, I had it bored and put the next size piston in, but now with the plated cylinders I'm not sure if slight scuffing is acceptable or what the rule of thumb is. The crank turns smooth and there isn't much rod play, but I do see a slight blue color on one part of the crank as you roll it. The rod says "Hot Rods" on it, so it's probably not the original bottom. I sure hate the thought of taking the whole bottom apart. Is it a given that the bottom is damaged when a piston breaks like this? Thanks for any help.
Steve
Steve
